We are studying a genetic element in Drosphila called the P factor, which is known to cause a syndrome of germaline abnormalities triggered by certain interstrain crosses. These abnormalities, known collectively as hybrid dysgenesis, include partial sterility, mutability, and chromosome breakage. Our aim is to understand the genetic mechanism involved and its effect on individuals and populations. We are especially interested in the P factor because it appears to be an example of a movable element which is anemable to study a higher-than-molecular levels of organization. Our long-time goal is to use the P factor to learn more about the wider problem of movable elements in higher organisms. Our specific short-term objectives are (1) providing further tests of the hypothesis that the F factor is a movable element, (2) describing the distribution of the P factor in individual genomes and in populations, and (3) obtaining further descriptive data on the mode of action and genetic control of the P factor.